In the low temperature processing of hydrocarbon feedstreams, e.g., liquefaction of natural gas, it is often necessary to reduce the concentration of water and carbon dioxide in the feedstream prior to any low temperature processing in order to prevent solidification of those components, commonly known as freeze-up. Some typical low temperature processing operations wherein such purification is required include nitrogen rejection, helium recovery, liquified natural gas processing operations and deep ethane recovery.
It is common practice in processes such as disclosed above to utilize molecular sieve dehydration to meet the low dew point requirements in the low temperature gas processing facilities which recover hydrocarbons. It is also common practice to employ liquid absorption processes such as those using solvents containing alkanol amines to remove carbon dioxide from the feedstreams. For example, U.S. Pat. No. 4,702,898, issued to Grover discloses a process for the removal of acid gases from mixtures which utilizes an alkaline scrubbing solution to remove the acid gases, e.g., carbon dioxide, from the gas mixtures. In addition to acid gas absorption, solid absorbents, e.g., molecular sieves, can be employed for the further removal of carbon dioxide depending upon the ability of the liquid absorption system to remove carbon dioxide and upper limits on the permissible carbon dioxide concentration. For example, adsorption is often employed when it is necessary to substantially remove carbon dioxide to levels of about 50 to 200 ppmv carbon dioxide, such as is typically required in liquefaction or deep ethane recovery. In some instances, it can be desirable to eliminate the liquid carbon dioxide absorption unit and perform the carbon dioxide removal by molecular sieve adsorption along, e.g., for purification where bulk carbon dioxide removal is not required.
In typical low temperature hydrocarbon processing operations where adsorbents are used to remove water and carbon dioxide, both components are removed in the same adsorber bed. Note, for example, U.K. Patent Application GB 2 181 667A published Apr. 29, 1987 which discloses a process wherein both carbon dioxide and water vapor are removed from a natural gas feed in an adsorber bed. However, adsorption systems which are designed to remove both water and carbon dioxide in a single adsorber bed can be plagued with major problems. For instance, it is known that water is a strongly adsorbed component whereas carbon dioxide is much less strongly adsorbed. Hence, since adsorption is favored at lower temperatures, it is typically desirable to perform the carbon dioxide adsorption at the lowest temperatures possible. However, the temperatures are limited by the formation of hydrates in the presence of water at low temperatures which can cause plugging or blockage problems. Hydrates can be defined as solid materials formed by the combination of a hydrocarbon with water, usually at low temperatures. Similarly, because water is more strongly adsorbed than carbon dioxide, it is usually desirable to regenerate the water-loaded adsorbent at a substantially higher temperature than is necessary to regenerate the carbon dioxide-loaded adsorbent. Moreover, at typical regeneration temperatures for regenerating water-loaded adsorbent beds, e.g., above 500.degree. F., the hydrocarbons present in the adsorber bed can be converted to carbon oxides and water when there is oxygen present in the regeneration gas. Thus, there can be a built-up of carbon oxides and water in the system. This type of problem is addressed in U.S. Pat. No. 4,025,321, issued to Anderson et al.
An adsorption system having separate dehydration and carbon dioxide adsorber beds has been proposed. U.S. Pat. No. 3,841,058, issued to Templeman, discloses a method of purifying natural gas or the like to render it suitable for liquefaction. The method consists essentially of adsorbing water and methanol from a stream of natural gas containing water, methanol and carbon dioxide in a first bed of an adsorbent material and subsequently adsorbing the carbon dioxide in a second bed of adsorbent material. The first adsorber bed is regenerated by passing a gas therethrough at an elevated temperature, i.e., thermal swing adsorption. The second adsorber bed is regenerated by reducing the pressure within the bed and also by passing a gas therethrough at a low temperature to displace desorbed carbon dioxide from the adsorber bed, i.e., a pressure swing adsorption cycle. The patent discloses that the adsorption effluent gas from the first adsorber bed can be cooled to subambient temperatures to increase the adsorptive capacity of the molecular sieves for carbon dioxide.
The method disclosed in above-identified U.S. Pat. No. 3,841,058, however, does not provide an adequate solution to the problem of removing water and carbon dioxide prior to low temperature processing. More specifically, because the second adsorber bed is regenerated by pressure swing adsorption, there is inherently less hydrocarbon recovery due to the fact that pressure swing cycles are usually operated at a shorter cycle time than thermal swing cycles, e.g., minutes versus hours, and hence, the hydrocarbon feed gas which remains in the void space after the adsorption step is terminated is lost in the desorption effluent stream when the adsorber bed is depressurized. In addition, because thermal swing adsorption cycles typically provide more complete regeneration than is generally possible with pressure swing cycles, higher residual carbon dioxide levels are present on the adsorbent subjected to pressure swing regeneration. The higher residual levels cause higher levels of carbon dioxide in the product gas since the concentration of carbon dioxide in the product gas is in equilibrium with the carbon dioxide adsorbed on the adsorbent in the effluent end of the adsorber bed. In order to keep the carbon dioxide content of the adsorbent low in the effluent end of the adsorber bed it is necessary to reduce the cycle time, however, reduced cycle times contribute to the recovery losses described above. Hence, the above-identified patent describes a process that is deficient due to the use of the pressure swing adsorption cycle in the second adsorber bed as compared to a thermal swing cycle.
As noted above, nitrogen rejection operations are one of the low temperature processes which often require the removal of water and carbon dioxide from the hydrocarbon feedstream before it can be further processed. Hydrocarbon fractions obtained from oil and gas wells often contain nitrogen which can be present naturally or as a result of petroleum production methods which frequently utilize high pressure nitrogen injection to maintain well head pressure for enhanced oil and gas recovery. In enhanced recovery operations as nitrogen is injected, the natural gas from the well containing methane and associated hydrocarbon liquids also contains nitrogen which typically increases in amount over the life of the nitrogen injection project to levels above pipeline specifications. Also, it is not uncommon for naturally occurring nitrogen levels to be above pipeline specifications. For these reasons, natural gas which contains nitrogen must be separated to reject the nitrogen and form purified natural gas feedstreams suitable for utilization as fuel or chemical feedstreams. It can also be desirable to reject nitrogen to avoid pricing penalties applied to nitrogen-containing natural gas feedstreams.
In nitrogen rejection units, low temperature distillation is typically employed to separate the nitrogen fraction from the natural gas. Both single and multi-column arrangements have been proposed to accommodate varying nitrogen contents in the raw natural gas. For example, U.S. Pat. No. 4,504,295, issued to Davis et al., discloses a process for the recovery of methane, nitrogen and natural gas liquids from a natural gas feedstream which utilizes a nitrogen rejection stage including a heat pump driven distillation column and a natural gas liquid stage. This patent, for example, discloses at column 5, lines 29-31, that the natural gas feedstream is subjected to bulk carbon dioxide removal and drying. U.S. Pat. No. 4,662,919, issued to Davis, discloses a process for separating or rejecting nitrogen from a natural gas feedstream using a single distillation column. In addition to the above-identified patents, various aspects of nitrogen rejection processing operations including the dehydration and carbon dioxide removal stages are disclosed in the following publication. Lugosch, Economies of Scale in Small Nitrogen Rejection Units, Proceedings of the Sixty-Fifth Annual Convention, Gas Processors Association, 1986, at pages 96-104.
Helium recovery is also becoming an important low temperature process because helium is considered to be a valuable commodity. Helium can be recovered in a similar fashion to the rejection processes described above. As with nitrogen rejection processes, it is often necessary in helium recovery operations to remove carbon dioxide and water from the feedstreams to low levels.
Since water and carbon dioxide removal are important and necessary aspects of low temperature hydrocarbon processing operations such as for nitrogen rejection, improved processes are sought which can utilize adsorption technology for both water and carbon dioxide removal. Moreover, processes are sought which can provide improved recovery of the hydrocarbons in the feedstream and also provide desorption effluent streams, i.e., containing water and carbon dioxide, that can be conveniently passed to a pressurized pipeline downstream of the low temperature processing operation without the need for excessive compression.